My 125 Riva also has a lean surge from 35 to 45. It even occurs during full acceleration, but to a lesser extent. It also has some odd high at times idle characteristics combined with extremely low idle at other times. The carb has been well cleaned, set up per the service manual and a fuel filter is installed. The surge begins when the engine is fully warmed. I've yet to figure if its is related to a vacuum leak. The carb adapter may leak at the head when warm.
The carb is very simple and relies on the main jet for the entire useful rev range. E10 fuel may be making a lean designed system too lean. I will get a can of no ethanol fuel later and see if the problem resolves itself. That will be after the warmed engine is tested for vacuum leaks. After that, the float level will be raised to see if a bit more weight will reduce the issue.

You could buy a second one, or you could look for a Honda CH150. They are watercooled, and a bit bigger than the Yamaha 125, but they are more reliable in a lot of ways. Yamaha has always had trouble getting the carburetion right on the Riva. Look for 85, 86 or 87 Honda Elite 150.

Everyone has their favorites on a forum like this one, and mine is Honda scooters from the mid 80's. The 250 is even more powerful, but costs a bit more, too. Maybe only a couple of hundred dollars.

Make sure the vaccuum petcock is working properly, it may be running out of fuel at high rpm . Also check float level for richness problem and irratic idle. I just bought a 150 Elite with 18k on it and what a sweet running scoot, I swear it will out run my 250 Elite up to 60mph.

I got a used un-molested carb off of eBay, came in on Friday. Installed it. Run it. And it did the same thing as the original carb.
I feel now the engine is running too rich when it is surging and eventual shut down. Or oil is finding it's way onto the spark plug and killing it. After the engine is dead, I would wait for 3 to 5 minutes for restart. Then, engine starts OK, as if the plug needed the time to dry off.

Reading "cheap bastard"s post makes me to think this is a defect. The second carb installed did exactly what the "bastard" says including high and low idle speed.

There was a write up on CV carbs saying that on these carbs, pilot jet is supplying fuel up to 3/4 of the way. Rather than main jet.

Vacuum leak has been checked ok,even even when hot. Fuel supply to carb has been verified good while surging is going on. I have a large see-through fuel installed.

The guy who was selling his Riva has not returned my call, so I don't think I am getting that one. Which is OK with me.

I like Honda. I own three Honda cars. There are quite a few Metropolitan available but it's too small/slow for me. Elite 150? I'v never seen it around here. There was Elite 250 up for sale a while ago. I may go for this one if it is still there.

The Riva and I went on a 75 mile round trip to Blackhawk Farms raceway yesterday. with no traffic to contend with, it was easy to concentrate on the surging issue. The engine seems to have a changing load against it. Maybe its not fuel or even engine related at all. As kids we worked on and beat old mini bikes, crude go karts and old under powered snowmobiles. The centrifugal clutches were often worn out or replaced with cheap low quality parts. They sometimes acted in a similar fashion. There was even drivetrain squeal when chains were used. My riva has a new Gates belt, but there is drivetrain squeal. When the that occurs, the surge is greatly reduced. If there is an incline, strong headwind or need for quicker acceleration, the problem is greater. The oldest boy weighs about 80 lbs less than me and has no surge complaint on the bike. Any slight decline or strong tailwind results in no surge.
The clutch may be glazed or the friction material worn out. I will be pricing an upgraded clutch such as the Dr Pulley unit before taking the gearbox apart to look at it. I guess while its apart, a new set of roller weights or the lighter slides may get put to use.

I had a elite 80 that did the exact same thing and after weeks of fuckin with the carb I finally removed an inline fuel filter and it worked perfect. I couldn't belive it but the reason I did it was because everytime it started acting strange I noticed the filter looked empty. I even replaced the petcock, tried running without the tank cap on and who knows what else untill I hooked up my little fuel bottle I use for syncing carbs and it ran fine. I guess the inline filter along with the filter in the petcock was too much resistance for the gravity feed to overcome.

I didn't work on my Riva this weekend except pulling the spark plug off after it surged a lot. Did what they call "plug chop".

Well the plug was wet, indicating the mixture is too rich. One thing I can think of that causes surging like we are experiencing would be fuel level not steady during operation. I think I'll order new needle valve for the carb. Iridium plug is on the way
to help on combustion department.

I don't hear squeal from transmission or engine.

Fuel supply to my carb is good. Tested with drain valve open during cranking.

The iridium plug came in and installed. Ran a little better with #2 carb (bought off of eBay, used) but the issue is still there. = Loss of power in mid range of throttle.

So, removed the #2 carb and cleaned by soaking in solution this time since I didn't do it as I got from eBay. Checked and blow clean all jets and passage. Checked float level and was within the spec. Auto choke plungers were a little sticky before but are good now. Auto choke heat element is checked and is working as it should. Needle valve is in good condition with minimum sign of wear.

Checked valve clearances (intake and exhaust) and were fine, within spec.

Installed this #2 carb and test run after re assembly. Engine starts fine and rev fine on stand. Just like it has been. But, ran like crapoo-poo after warm up. Just like it has been. The spark plug is wet.

A friend of mine said it must be the float level which is too high and needs to be adjusted. He took #1 carb (came with the scoot) and adjusted float level to eyeballed height. He rebuilds engines and such for living, so it's not like he doesn't know about the stuff. Installed the #1 carb and engine starts fine. Rev fine on stand. Just like it has been. But, run like poo poo on tarmac.

My brother, in my home town, Suzuka, Japan who ran an auto/motorcycle repair shop says there's not much adjustment be made on these TK cv carbs. Clean and make sure all the passage is clear, he says. Which I think I have done so many times. He also says it is possible the ignitor module is is bad. New igniter is available but cost me almost $350 plus shipping. Used ones are on eBay but they are not so cheap either, especially I don't know if I need the replacement or not. Engine revs to higher RPM on the stand.

The purchase of this scoot was to save gas money. It's not working out as I planed at all thus far.

As the belt has "worn in" the driveline squeal has subsided for the time being. The bog was still there and nearly stalled the engine, when it happened at 50+ mph after having very little surge accelerating. The problem occurred a few more times over the next couple days. This morning I was looking for a problem with the small lawn mower fuel filter or the petcock before removing it to inspect the screen. The vacuum hose from the carb to the petcock was collapsing when the engine was running! The part was too soft for a vacuum hose. Maybe this is a cheesy replacement? A 5 inch piece of 3/16" Bundy flex, (brake line) was bent to replace all but 5/8" of the hose. A slight flare to retain the now short connection hoses and the problem was corrected. A 14 mile trial run at 35- 50 mph suburban riding resulted in...... no surge. Its in the low 90s, air is dry with air pressure over 30.5, the sort of conditions that accentuated the issue. I don't know if its solved till I ride a few days, but I'm having a Leinie's to toast the appearance of success.

Cool Riva are supost to be a bitch when it comes to the carb issues. Let hope it stays this way.

__________________
Life's journey is not to arrive at the grave safely in a well preserved body,but rather to skid in sideways totally worn out shouting WHAT A RUSH, WHAT A RIDE.
"Life is a tragedy when seen in close-up, but a comedy in long-shot." Charlie Chaplin

I was watching fuel line (clear) when the engine was running. It appears that fuel feeding into carb has pulsation. Back flowing at times. This sure has effect on fuel metering. I want correct it but I don't know how this is happening.

These holes are supplying extra/un-metered fuel into engine and the engine was choking on it. I ordered new valve assembly for $55 with shipping. Ouch. Meanwhile, I will see if I can rig up fuel system so I can ride.

Test rode around a few miles with this set up and it runs good now. Wasted a lot of time and $$ but learned a lot along the way. Finally I figured out the problem, which is priceless to me. For those who thinks it's got bad carb, it may not be the carb after all.

Now I can move on to other issue with this scooter, like replacing starter clutch and replacing tires.

After everything is put back together I had couples of errands with scooter. It performed well so I decided to use it to go home from work. Sky was clear, too. Night time in south Florida, flat roads, few traffic lights, I was doing 50ish MPH most of times.

Then, it happened. About 25 to 30 minutes into riding, the engine stumbled and lost power. Pulled back on throttle and it idled OK, with no funny sound. Apply the throttle and engine died. Immediate thought came to my mind, "overheat". Pushed scooter for about one hour heading back to my work place, I tried to start it. It started. This time, I'm doing 35 MPH. Almost back to my hangar, it started to loose power again. Shut down the engine and pushed it the rest of way back.

This morning, checked float level and is at 27mm, right on the spec. Checked idle mixture (pilot) screw. Factory setting was 2.5 turns out. I had it at 2.375ish out when I thought I had it at 2.5 turns out.

Now do you think I had it too lean?

Forced cooling air is flowing good and there's no gap in cooling shroud that I can see.

Pilleri SL26s were delivered Thursday. Had rims removed and took them to local bike shop (Seaside Powersports in 33462) for mounting. Any tire would have been good considering the tires this scooter had.

I rode the scooter to home last night and I have better idea of what happened the other night when I thought I had overheat problem. On the way home, the engine had lost drive/power in less than 10 miles. Engine does not quit if I let it idle. Stopped on the side of road. Adjusted mixture rich. Started riding again, only to have power loss in couple of miles. This time, I had a look at inline fuel filter with flashlight. It had a lot of air. Also, there's some more air in the tube leading into the carb. The carb was starving for fuel in drive!!

This morning, I removed brass fuel valve and test rode the scooter. It kept on going for 20 miles with no problem. Checked valve operation on test bench and is working fine. So, I think it was extra long tube in funky routing caused fuel starvation. I cannot wait for new OEM vac operated fuel valve to arrive so the scooter would run reliably.